1. Field of the Invention
This invention generally relates to a hold-down device on handling machines, in particular punching machines, for thin, flat objects, in particular sheets of paper. The handling machine includes several handling stations located at intervals in a series, through which the sheets can be fed in steps by means of an endless conveyor, in particular a feed or conveyor chain, by means of grippers. After an acceleration phase and a deceleration phase, the sheets remain in the respective handling station during a rest phase, whereby the sheets arrive at a delivery station after passing the final handling station.
2. Background Information
Generally, hold-down devices of the type described above are intended to prevent or eliminate, by smoothing, the warping, wrinkling, waving or folding of the sheets concerned. Such warping, wrinkling, waving and folding can tend to occur at high production speeds during the deceleration phase, in particular ahead of the delivery station, and can thus interfere with correct processing and even result in interruptions of operation. To accomplish smoothing, the following arrangements have been used: brake brushes; smoothing blocks; or blowing air currents acting on the sheet from above. Of course, each of the arrangements just listed may produce the desired smoothing effect, but, particularly when sheets of sensitive or delicate material are used, those arrangements tend to have the disadvantage that damage can be caused to the upper side of the sheet, or that the sheet can tear on account of the relatively large friction forces between the underside of the sheet and the sheet feed surface.
In the general field of handling machines for flat objects, in particular sheets of paper, German Laid Open Patent Appln. No. 39 03 389, or European Patent Appln. No. 0 075 685, disclose the use of endless belts provided with air passages, or openings. As disclosed in those patent applications, the upper strand of such a belt forms the support for the sheets, and the underside of the belt is guided over a suction box equipped with suction openings. These devices are used for the transport of the sheets by a system operating without conveyor chains, feed chains, or similar devices, but not for the smoothing of the sheets during deceleration in systems equipped with conveyor chains. Accordingly, essentially no synchronization is provided.